1. Field of the Invention
The present invention relates to an image signal processing apparatus for receiving an analog image signal from an apparatus for producing an image signal, such as a CCD (charge coupled device), processing the received image signal, and converting the processed signal to a digital signal, as well as an image reading apparatus for reading image information on an original document by using the image signal processing apparatus and an image forming apparatus for forming an image on the basis of the read image information.
2. Description of the Related Art
Recently, CCDs with photoelectric converter element arrays have been widely used in image reading apparatuses and image reading units in a facsimile apparatus or a digital copying machine. In these apparatuses, a demand for high-speed processing, high-quality image formation, and color image formation has increased more and more, and special attention has been paid to the importance of signal processing for outputs of CCDs. Under the circumstances, various LSIs (large scale integration circuits) for signal processing have been developed. For example, as disclosed in Published Unexamined Japanese Patent Application Nos. 63-211876 and 4-100378, an LSI for image signal processing has been developed, which includes a clamping circuit for adjusting an offset voltage of an analog image signal output from a CCD, a sampling and holding circuit for sampling/holding the respective pixel signals included in the image signal for a predetermined timing, and an A/D converter for converting the sampled/held pixel signals to a digital signal.
In these LSIs, however, fine adjustment of the offset voltage of the clamping circuit, as well as adjustment for attenuating or amplifying a signal amplitude of the analog image signal and optimizing the signal amplitude in an input range of the A/D converter, is manually performed by using a variable resistor, etc. In addition, such adjustment requires a measuring device such as an oscilloscope and, consequently, requires a considerable amount of time. Moreover, such adjustment lacks precision and it is difficult to make the performance of each device after uniform adjustment.
Besides, when CCDs are connected to such a conventional LSI, a certain type of CCD requires an additional circuit. Specifically, two-channel output type CCDs include in-phase output type CCDs wherein a phase difference between output signals of the two channels is 0.degree., and opposite-phase output type CCDs wherein a phase difference between output signals of the two channels is 180.degree.. When such different types of CCDs are used, a circuit for matching a timing, such as an analog delay line, needs to be provided outside the LSI. In addition, in the case where a CCD which generates a positive-polarity image signal and a CCD which generates a negative-polarity image signal are used, a circuit for inverting the polarity of outputs needs to be provided outside the LSI.